Burner In Center
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On conventional gas- or oil- fired water heaters, heat is generated by a burner at the bottom of the tank. The tank is like a very tall doughnut, with a hole up through the central part of the tank. The exhaust products from the burner go up through this core and heat is transferred through the tank walls from the core into the water. The exhaust products go into a vent connector above the core and into a chimney or vent system.
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No Separate Heat Exchanger
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Domestic water heaters transfer heat directly from the combustion products through the tank wall and floor into the water. The tank is both the storage compartment and the heat exchanger. Most gas and oil domestic water heaters are center flue type. All of the combustion products go up through the center of the water heater. External flue water heaters are also available, although they are not common on single family homes.
Oil-fired water heaters may be external or floating tank systems. This means that the combustion products pass over the entire bottom of the tank and through flue passages on the outside of the tank (between the tank and the outer wall of the water heater).
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Ceramic Refractories
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Combustion chambers in modern oil water heaters are often ceramic fiber. These combustion chambers are light and help reduce the weight of the water heater while providing for adequate heat control and heat transfer. These combustion chambers can typically be replaced if necessary without replacing the entire water heater.
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Dip Tube
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Cold water is introduced through a dip tube near the bottom of the tank. The cold water supply pipe usually attaches to the tank at the top. The dip tube carries the cold water down through the tank, discharging it near the bottom of the tank. The hot water is drawn off from the top of the tank.
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Sacrificial Anode Rods
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Sacrificial anodes are typically magnesium or aluminum. The anode material that is most suitable is a function of the water chemistry. The idea is that corrosive chemicals in the water will attack the anode rod, rather than the tank itself. In some areas, these rods are replaced regularly. In other areas, the original rod is left in through the life of the water heater. The rate of deterioration of the anode is a function of both water chemistry and the water temperature in the tank. The higher the water temperature in the tank, the faster any chemical reactions will take place. Water heaters that keep water at 150°F will fail sooner than tanks with water stored at 120°F. Some studies show that the difference in life expectancy is dramatic and that changing the water temperature from 140°F to 160°F will result in a life expectancy for the 160°F tank that’s only about 60 percent of the life expectancy for the 140°F tank.
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Thermostat Control
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At rest, the tank is filled with water heated to the set temperature (e.g., 120°F). As water begins to be drawn off, the 120°F water leaving the tank is replaced by 50°F water. This cool water activates the thermostat which tells the burner to come on. The burner comes on and heats the water. Depending on how quickly the water is flowing out, the temperature in the tank may rise toward the set temperature, or may continue to drop because cold water comes in faster than the burner can heat it up.
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Cold Showers
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All of us have experienced what happens when water is drawn off faster than the burner can add heat. The shower water gets very cold. Water still flows through the water heater and the pressure is just fine, but the water has been moving through too quickly for the burner to heat it up. Once we stop flowing water, the burner can heat up the water in the tank and it is ready to use again. While we are sleeping the water in the tank cools, and the burner comes on briefly to reheat the water to its set point.
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Size Issues
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The effective size of a water heater is a function of —
- the volume of hot water it can keep on hand (capacity)
- the size of the burner (recovery rate)
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The larger the burner, the faster the recovery rate of the heater. The recovery rate is the time it takes to heat the tank full of water from 50°F to 140°F, for example. (Current conventions measure recovery rates over a 90° F temperature rise, even though most of us don’t heat water to 140° F anymore.) Generally speaking, the faster the recovery rate, the smaller the tank can be. Oil-fired water heaters typically have larger burners than gas-fired water heaters. Oil water heaters have a faster recovery rate, so their tanks are often smaller than gas water heater tanks.
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